Gnathiferans and Smaller Lophotrochozoans

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Gnathiferans and Smaller Lophotrochozoans

• Chapter 15

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Pseudocoelomates

• Pseudocoelomates have a body cavity (the
  pseudocoel) between the gut (derived from
  endoderm) and body wall (derived from mesoderm).
• Derived from the blastocoel.

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Advantages of a Body Cavity

• A body cavity, pseudocoel or coelom, has several
  advantages. It provides
• Greater freedom of movement.
• Space for development of organ systems.
• A simple means for circulation of materials
  around the body.
• Storage place for waste products.
• A hydrostatic organ.

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Common Features of Pseudocoelomates

• All have a body wall of epidermis, a dermis, and
  muscles surrounding the pseudocoel.
• Almost all have a complete digestive tract.
• The epidermis of many secretes a nonliving
  cuticle with bristles spines.

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Pseudocoelomates

• Pseudocoelomates do not form a clade.
• Some are part of superphylum Lophotrochozoa,
  others are in superphylum Ecdysozoa.
• All share the pseudocoelomate body plan.

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Lophotrochozoa (10 Phyla)

• Ancestors possessed complex cuticular jaws
  Clade Gnathifera
• Gnathostomulida
• Micrognathozoa
• Rotifera
• Acanthocephala
• 6 other lophotrochozoan phyla
• Gastrotricha
• Tiny aquatic animals that may be closely related
  to gnathiferans
• Molecular characteristics place the following
  with Lophotrochozoa
• Cycliophora
• Entoprocta
• Ectoprocta
• Brachiopoda
• Phoronida

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Clade Gnathifera

• Possess small cuticular jaws with a homologous
  microstructure.
• Numbers of pairs of jaws vary.
• Gnathostomulida, Micrognathozoa, and Rotifera are
  tiny, free-living, aquatic animals.
• Acanthocephalans are worm-like endoparasites
  living as adults in fish or other vertebrates.

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Clade Gnathifera

• Rotifera and Acanthocephala
• Presumed sister taxa.
• Form a clade called Syndermata.
• Have eutelic syncytial epidermis.
• Constant number of nuclei.
• Grouping is controversial.

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Phylum Gnathostomulida

• Phylum Gnathostomulida includes the jaw worms.
• Very small - lt2mm.
• Live in interstitial spaces of fine coastal
  sediments.
• Can endure low O2.

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Phylum Gnathostomulida

• Feed by scraping bacteria and fungi from the
  substratum with a pair of jaws on the pharynx.
• Acoelomate
• Sexual stages include males, females, and
  hermaphrodites.
• Fertilization is internal.

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Phylum Micrognathozoa

• Micrognathozoans are tiny animals that live
  interstitially (between sand grains).
• Body consists of a two-part head, a thorax, and
  abdomen with short tail.
• Move using cilia and have a unique ventral
  ciliary adhesive pad that produces glue.
• Three pairs of complex jaws.

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Phylum Micrognathozoa

• Simple gut
• Anus opens to outside only periodically.
• Reproductive system is not well understood
• Only female reproductive organs have been
  identified.
• May reproduce parthenogenetically.
• Cleavage and subsequent development have not been
  studied.

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Phylum Rotifera

• Members of the phylum Rotifera are
  pseudocoelomate and have three embryonic germ
  layers (triploblastic).
• Complete digestive system.

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Phylum Rotifera

• Dioecious (separate sexes) but some species
  parthenogenetic (females produce diploid eggs).
• Some are parthenogenetic during part of the year,
  depending on environmental conditions.
• Thick shelled eggs that can withstand harsh
  conditions are sometimes produced.

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Phylum Rotifera

• Rotifers have a ciliated crown, the corona, that
  is characteristic of the phylum.

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Phylum Rotifera

• Rotifers come in a wide range of colors and
  shapes.
• Shapes often correspond to lifestyle (floaters,
  swimmers, sessile).
• They may be individual or colonial.
• Mostly freshwater.
• Benthic and pelagic forms.

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Phylum Acanthocephala

• All spiny-headed worms (Phylum Acanthocephala)
  are parasites in the intestines of vertebrates.
• Over 1100 species known.
• Occur worldwide and parasitize fish, birds, and
  mammals.
• Larvae develop in crustaceans or insects.

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Phylum Acanthocephala

• Proboscis has rows of recurved spines that
  penetrate and may rupture host intestines.
• Proboscis with hooks can be inverted into a
  proboscis receptacle by retractor muscles.

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Phylum Acanthocephala

• Body somewhat flattened.
• About 80 of tegument is a lacunar system of
  fluid-filled canals that may distribute nutrients
  and remove wastes from muscles.
• No heart - function provided by lacunar fluid.
• Both longitudinal and circular body wall muscles
  are present.

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Phylum Acanthocephala

• No respiratory system.
• Protonephridia with flame cells, if present,
  perform excretory functions.
• Nutrients are absorbed across the tegument, which
  bears some enzymes - no digestive tract.
• Dioecious
• No species normally parasitizes humans.

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Phylum Acanthocephala

• Acanthocephalans penetrate the intestinal wall
  with spiny proboscis.
• Remarkably little inflammation on host wall, but
  pain of infection is intense.
• Larval acanthors burrow through beetle intestine.
• Develop into juvenile cystacanths in the insect
  hemocoel.
• Pigs become infected by eating grubs.

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Phylum Gastrotricha

• Gastrotrichs appear similar to rotifers, but
  without the ciliated corona and have a bristly
  looking body.
• Members of the phylum Gastrotricha are
  pseudocoelomate and have three embryonic germ
  layers (triploblastic).
• Complete digestive system.
• Hermaphroditic or parthenogenetic.

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Phylum Entoprocta

• About 150 species in the phylum Entoprocta occur
  worldwide.
• Usually in marine environments.
• Less than 5 mm long and mostly microscopic,
  resembling hydroid cnidarians.

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Phylum Entoprocta

• Urnatella gracilis is a common freshwater species
  in North America.
• Body or calyx is cup shaped and bears a circular
  crown of ciliated tentacles.
• Attaches by a stalk with adhesive glands.

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Phylum Entoprocta

• Tentacles (3-30) and stalk are continuations of
  the body wall.
• Tentacles on lateral and inner surfaces can roll
  inward but cannot be retracted into the calyx.
• Gut is U-shaped with both mouth and anus opening
  within the circle of tentacles.

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Phylum Entoprocta

• Long cilia on sides generate current bringing in
  particles.
• Short cilia on inner surfaces capture food and
  direct it to mouth.
• Pair of protonephridia embedded in gelatinous
  parenchyma.
• Well-developed nerve ganglion on the ventral side
  of stomach.
• No circulatory or respiratory organs.

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Phylum Entoprocta

• Some are monoecious, some dioecious, and some
  first produce sperm and later eggs.
• Fertilized eggs develop in a brood pouch.
• Modified spiral cleavage leads to
  trochophore-like larva.

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Lophophorates

• Phylum Ectoprocta
• Phylum Brachiopoda
• Phylum Phoronida
• Belong within the lophotrochozoan subgroup of
  protostomes.
• Grouping very controversial.
• Evidence comes from sequence analysis of genes
  encoding small-subunit ribosomal RNA.
• Some aspects of development place these taxa
  within Deuterostomia.

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Lophophorates

• Lophophorates possess a mesoderm-lined coelom.
• Tripartite coelom coelom is divided into three
  parts protocoel, mesocoel, metacoel.

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Lophophorates

• Members of these 3 taxa possess a feeding device
  called a lophophore.
• Unique arrangement of ciliated tentacles borne on
  a ridge or fold on the body wall.
• Tentacles are hollow and contain an extension of
  the mesocoel.

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Lophophorates

• Ciliated tentacles are also a respiratory device
  permitting gas exchange between surrounding water
  and internal coelomic fluid.
• Gut is U-shaped
• Mouth opens inside the lophophore ring, and the
  anus opens outside the ring.

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Phylum Ectoprocta

• Phylum Ectoprocta contains aquatic animals that
  often encrust hard surfaces (bryozoans).
• Approximately 4500 living species.
• Inhabit both shallow freshwater and marine
  habitats.
• Most are sessile, some slide slowly, and others
  crawl actively across surfaces.
• Mostly colony builders.
• Each member is less than 0.5 mm in length and is
  called a zooid.

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Phylum Ectoprocta

• Zooids feed by extending lophophores into
  surrounding water to collect tiny particles
• Zooids secrete exoskeleton in which they live in.

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Phylum Ectoprocta

• Exoskeleton may be gelatinous, chitinous, or
  stiffened with calcium and possibly impregnated
  with sand.
• Shape may be boxlike, vaselike, oval, or tubular.
• Some colonies form limy encrustations on seaweed,
  shells, and rocks.
• Others form fuzzy or shrubby growths or erect
  branching colonies.
• Freshwater colonies may form mosslike colonies on
  stems of plants or on rocks.

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Phylum Ectoprocta

• To feed, the lophophore is extended and
  tentacles spread out into a funnel.
• Cilia on tentacles draw water into funnel.
• Food particles caught by cilia in the funnel are
  drawn into the mouth.
• Digestion begins extracellularly in the stomach
  and is completed intracellularly within the
  intestine.

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Phylum Ectoprocta

• Respiratory, vascular, and excretory organs
  absent.
• Gas exchange is through body surface.
• Ganglionic mass and a nerve ring around the
  pharynx.
• No sense organs.

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Phylum Ectoprocta

• Reproduction - most hermaphroditic.
• Some species shed eggs into seawater, but most
  brood their eggs.
• Brooding occurs within coelom and some have an
  external chamber called an ovicell.
• Sometimes embryos proliferate asexually from the
  initial embryo.
• Cleavage is radial but mosaic.
• Larva of nonbrooding species have a functional
  gut and swim for a few months before settling.

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Phylum Ectoprocta

• Larva of brooding species do not feed and settle
  after a brief free-swimming existence.
• Attach to substratum by secretions from an
  adhesive sac, then metamorphose to adult form.
• New colonies begin from this single metamorphosed
  primary zooid, called an ancestrula.
• Ancestrula undergoes asexual budding to produce
  many zooids of a colony.

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Phylum Ectoprocta

• Freshwater ectoprocts undergo budding that
  produces statoblasts.
• Hard, resistant capsules containing a mass of
  germinative cells.

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Phylum Brachiopoda

• Brachiopods appear similar to bivalve molluscs
  because they have two calcareous shell valves
  secreted by a mantle.
• Dorsal/ventral instead of left/right.
• Pedicel a fleshy stalk used for attachment.

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Phylum Brachiopoda

• Brachiopods are an ancient group they were
  prolific during the Paleozoic and Mesozoic eras.
• One living species, Lingula, is considered to be
  a living fossil since it has changed little since
  the Ordovician (505 mya).

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Phylum Brachiopoda

• Characteristics of both protostomes
  deuterostomes
• Cleavage is radial (deuterostome)
• Coelom formation enterocoelous at least in some
  brachiopods. (deuterostome)
• The relationship of the blastopore to the mouth
  is uncertain.

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Phylum Phoronida

• Species in the phylum Phoronida are small
  wormlike animals.
• Secrete tubes to live in.
• Tentacles of the lophophore are extended for
  feeding.
• U-shaped digestive tract.

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Phylum Phoronida

• Characteristics of both protostomes
  deuterostomes
• Blastopore becomes mouth (protostome).
• Cleavage is radial (deuterostome).
• Coelom formation highly modified enterocoelous
  (deuterostome).

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Phylogeny

• Analysis of rRNA gene sequencing suggests that
  after the ancestral deuterostome diverged from
  ancestral protostomes
• Protostomes split into two large groups
• Ecdysozoa that molt.
• Lophotrochozoa that exhibit lophophore feeding
  and trochophore-like larvae.
• Most lophotrochozoans share some developmental
  features
• Spiral mosaic cleavage and formation of mouth
  from embryonic blastopore.
• No common body plan.

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Phylogeny

• Lophotrochozoan protostomes are a heterogeneous
  group for which evolutionary branching order
  remains to be determined.
• DNA sequence analysis has led to the conclusion
  that acanthocephalans are highly derived
  rotifers.
• Sequences put Acanthocephala and Rotifera
  together as clade Syndermata, sharing a eutelic
  syncytial epidermis.
• Syndermata is placed with Micrognathozoa and
  Gnathostomulida in clade Gnathifera.
• Controversial placement of Cycliophora,
  Gastrotricha, and Platyhelminthes close to
  Gnathifera.

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Phylogeny

• Entoprocts and ectoprocts were once considered
  both Bryozoa, but ectoprocts have been broken out
  as true coelomate animals.
• Sequence analysis now places them both in the
  lophotrochozoan phyla.

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Phylogeny

• Ectoprocts, brachiopods, and phoronids share a
  lophophore and a tripartite coelom.
• Other features are mixtures of developmental
  traits from both protostomes and deuterostomes.
• Debate continues on whether the lophophorates
  form a clade or whether the group members, either
  individually or collectively belong within
  Protostomia or Deuterostomia.